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<a name="Basic-Python"></a>
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<p>
Next: <a href="Exception-Handling.html#Exception-Handling" accesskey="n" rel="next">Exception Handling</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Basic-Python-1"></a>
<h4 class="subsubsection">23.2.2.1 Basic Python</h4>

<a name="index-python-stdout"></a>
<a name="index-python-pagination"></a>
<p>At startup, <small>GDB</small> overrides Python&rsquo;s <code>sys.stdout</code> and
<code>sys.stderr</code> to print using <small>GDB</small>&rsquo;s output-paging streams.
A Python program which outputs to one of these streams may have its
output interrupted by the user (see <a href="Screen-Size.html#Screen-Size">Screen Size</a>).  In this
situation, a Python <code>KeyboardInterrupt</code> exception is thrown.
</p>
<p>Some care must be taken when writing Python code to run in
<small>GDB</small>.  Two things worth noting in particular:
</p>
<ul>
<li> <small>GDB</small> install handlers for <code>SIGCHLD</code> and <code>SIGINT</code>.
Python code must not override these, or even change the options using
<code>sigaction</code>.  If your program changes the handling of these
signals, <small>GDB</small> will most likely stop working correctly.  Note
that it is unfortunately common for GUI toolkits to install a
<code>SIGCHLD</code> handler.

</li><li> <small>GDB</small> takes care to mark its internal file descriptors as
close-on-exec.  However, this cannot be done in a thread-safe way on
all platforms.  Your Python programs should be aware of this and
should both create new file descriptors with the close-on-exec flag
set and arrange to close unneeded file descriptors before starting a
child process.
</li></ul>

<a name="index-python-functions"></a>
<a name="index-python-module"></a>
<a name="index-gdb-module"></a>
<p><small>GDB</small> introduces a new Python module, named <code>gdb</code>.  All
methods and classes added by <small>GDB</small> are placed in this module.
<small>GDB</small> automatically <code>import</code>s the <code>gdb</code> module for
use in all scripts evaluated by the <code>python</code> command.
</p>
<a name="index-gdb_002ePYTHONDIR"></a>
<dl>
<dt><a name="index-gdb_002ePYTHONDIR-1"></a>Variable: <strong>gdb.PYTHONDIR</strong></dt>
<dd><p>A string containing the python directory (see <a href="Python.html#Python">Python</a>).
</p></dd></dl>

<a name="index-gdb_002eexecute"></a>
<dl>
<dt><a name="index-gdb_002eexecute-1"></a>Function: <strong>gdb.execute</strong> <em>(command <span class="roman">[</span>, from_tty <span class="roman">[</span>, to_string<span class="roman">]]</span>)</em></dt>
<dd><p>Evaluate <var>command</var>, a string, as a <small>GDB</small> CLI command.
If a GDB exception happens while <var>command</var> runs, it is
translated as described in <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>.
</p>
<p>The <var>from_tty</var> flag specifies whether <small>GDB</small> ought to consider this
command as having originated from the user invoking it interactively.
It must be a boolean value.  If omitted, it defaults to <code>False</code>.
</p>
<p>By default, any output produced by <var>command</var> is sent to
<small>GDB</small>&rsquo;s standard output (and to the log output if logging is
turned on).  If the <var>to_string</var> parameter is
<code>True</code>, then output will be collected by <code>gdb.execute</code> and
returned as a string.  The default is <code>False</code>, in which case the
return value is <code>None</code>.  If <var>to_string</var> is <code>True</code>, the
<small>GDB</small> virtual terminal will be temporarily set to unlimited width
and height, and its pagination will be disabled; see <a href="Screen-Size.html#Screen-Size">Screen Size</a>.
</p></dd></dl>

<a name="index-gdb_002ebreakpoints"></a>
<dl>
<dt><a name="index-gdb_002ebreakpoints-1"></a>Function: <strong>gdb.breakpoints</strong> <em>()</em></dt>
<dd><p>Return a sequence holding all of <small>GDB</small>&rsquo;s breakpoints.
See <a href="Breakpoints-In-Python.html#Breakpoints-In-Python">Breakpoints In Python</a>, for more information.  In <small>GDB</small>
version 7.11 and earlier, this function returned <code>None</code> if there
were no breakpoints.  This peculiarity was subsequently fixed, and now
<code>gdb.breakpoints</code> returns an empty sequence in this case.
</p></dd></dl>

<a name="index-gdb_002eparameter"></a>
<dl>
<dt><a name="index-gdb_002eparameter-1"></a>Function: <strong>gdb.parameter</strong> <em>(parameter)</em></dt>
<dd><p>Return the value of a <small>GDB</small> <var>parameter</var> given by its name,
a string; the parameter name string may contain spaces if the parameter has a
multi-part name.  For example, &lsquo;<samp>print object</samp>&rsquo; is a valid
parameter name.
</p>
<p>If the named parameter does not exist, this function throws a
<code>gdb.error</code> (see <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>).  Otherwise, the
parameter&rsquo;s value is converted to a Python value of the appropriate
type, and returned.
</p></dd></dl>

<a name="index-gdb_002ehistory"></a>
<dl>
<dt><a name="index-gdb_002ehistory-1"></a>Function: <strong>gdb.history</strong> <em>(number)</em></dt>
<dd><p>Return a value from <small>GDB</small>&rsquo;s value history (see <a href="Value-History.html#Value-History">Value History</a>).  The <var>number</var> argument indicates which history element to return.
If <var>number</var> is negative, then <small>GDB</small> will take its absolute value
and count backward from the last element (i.e., the most recent element) to
find the value to return.  If <var>number</var> is zero, then <small>GDB</small> will
return the most recent element.  If the element specified by <var>number</var>
doesn&rsquo;t exist in the value history, a <code>gdb.error</code> exception will be
raised.
</p>
<p>If no exception is raised, the return value is always an instance of
<code>gdb.Value</code> (see <a href="Values-From-Inferior.html#Values-From-Inferior">Values From Inferior</a>).
</p></dd></dl>

<a name="index-gdb_002eparse_005fand_005feval"></a>
<dl>
<dt><a name="index-gdb_002eparse_005fand_005feval-1"></a>Function: <strong>gdb.parse_and_eval</strong> <em>(expression)</em></dt>
<dd><p>Parse <var>expression</var>, which must be a string, as an expression in
the current language, evaluate it, and return the result as a
<code>gdb.Value</code>.
</p>
<p>This function can be useful when implementing a new command
(see <a href="Commands-In-Python.html#Commands-In-Python">Commands In Python</a>), as it provides a way to parse the
command&rsquo;s argument as an expression.  It is also useful simply to
compute values, for example, it is the only way to get the value of a
convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>) as a <code>gdb.Value</code>.
</p></dd></dl>

<a name="index-gdb_002efind_005fpc_005fline"></a>
<dl>
<dt><a name="index-gdb_002efind_005fpc_005fline-1"></a>Function: <strong>gdb.find_pc_line</strong> <em>(pc)</em></dt>
<dd><p>Return the <code>gdb.Symtab_and_line</code> object corresponding to the
<var>pc</var> value.  See <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>.  If an invalid
value of <var>pc</var> is passed as an argument, then the <code>symtab</code> and
<code>line</code> attributes of the returned <code>gdb.Symtab_and_line</code> object
will be <code>None</code> and 0 respectively.
</p></dd></dl>

<a name="index-gdb_002epost_005fevent"></a>
<dl>
<dt><a name="index-gdb_002epost_005fevent-1"></a>Function: <strong>gdb.post_event</strong> <em>(event)</em></dt>
<dd><p>Put <var>event</var>, a callable object taking no arguments, into
<small>GDB</small>&rsquo;s internal event queue.  This callable will be invoked at
some later point, during <small>GDB</small>&rsquo;s event processing.  Events
posted using <code>post_event</code> will be run in the order in which they
were posted; however, there is no way to know when they will be
processed relative to other events inside <small>GDB</small>.
</p>
<p><small>GDB</small> is not thread-safe.  If your Python program uses multiple
threads, you must be careful to only call <small>GDB</small>-specific
functions in the <small>GDB</small> thread.  <code>post_event</code> ensures
this.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) python
&gt;import threading
&gt;
&gt;class Writer():
&gt; def __init__(self, message):
&gt;        self.message = message;
&gt; def __call__(self):
&gt;        gdb.write(self.message)
&gt;
&gt;class MyThread1 (threading.Thread):
&gt; def run (self):
&gt;        gdb.post_event(Writer(&quot;Hello &quot;))
&gt;
&gt;class MyThread2 (threading.Thread):
&gt; def run (self):
&gt;        gdb.post_event(Writer(&quot;World\n&quot;))
&gt;
&gt;MyThread1().start()
&gt;MyThread2().start()
&gt;end
(gdb) Hello World
</pre></div>
</dd></dl>

<a name="index-gdb_002ewrite"></a>
<dl>
<dt><a name="index-gdb_002ewrite-1"></a>Function: <strong>gdb.write</strong> <em>(string <span class="roman">[</span>, stream])</em></dt>
<dd><p>Print a string to <small>GDB</small>&rsquo;s paginated output stream.  The
optional <var>stream</var> determines the stream to print to.  The default
stream is <small>GDB</small>&rsquo;s standard output stream.  Possible stream
values are:
</p>
<dl compact="compact">
<dd><a name="index-STDOUT"></a>
<a name="index-gdb_002eSTDOUT"></a>
</dd>
<dt><code>gdb.STDOUT</code></dt>
<dd><p><small>GDB</small>&rsquo;s standard output stream.
</p>
<a name="index-STDERR"></a>
<a name="index-gdb_002eSTDERR"></a>
</dd>
<dt><code>gdb.STDERR</code></dt>
<dd><p><small>GDB</small>&rsquo;s standard error stream.
</p>
<a name="index-STDLOG"></a>
<a name="index-gdb_002eSTDLOG"></a>
</dd>
<dt><code>gdb.STDLOG</code></dt>
<dd><p><small>GDB</small>&rsquo;s log stream (see <a href="Logging-Output.html#Logging-Output">Logging Output</a>).
</p></dd>
</dl>

<p>Writing to <code>sys.stdout</code> or <code>sys.stderr</code> will automatically
call this function and will automatically direct the output to the
relevant stream.
</p></dd></dl>

<a name="index-gdb_002eflush"></a>
<dl>
<dt><a name="index-gdb_002eflush-1"></a>Function: <strong>gdb.flush</strong> <em>()</em></dt>
<dd><p>Flush the buffer of a <small>GDB</small> paginated stream so that the
contents are displayed immediately.  <small>GDB</small> will flush the
contents of a stream automatically when it encounters a newline in the
buffer.  The optional <var>stream</var> determines the stream to flush.  The
default stream is <small>GDB</small>&rsquo;s standard output stream.  Possible
stream values are: 
</p>
<dl compact="compact">
<dd><a name="index-STDOUT-1"></a>
<a name="index-gdb_002eSTDOUT-1"></a>
</dd>
<dt><code>gdb.STDOUT</code></dt>
<dd><p><small>GDB</small>&rsquo;s standard output stream.
</p>
<a name="index-STDERR-1"></a>
<a name="index-gdb_002eSTDERR-1"></a>
</dd>
<dt><code>gdb.STDERR</code></dt>
<dd><p><small>GDB</small>&rsquo;s standard error stream.
</p>
<a name="index-STDLOG-1"></a>
<a name="index-gdb_002eSTDLOG-1"></a>
</dd>
<dt><code>gdb.STDLOG</code></dt>
<dd><p><small>GDB</small>&rsquo;s log stream (see <a href="Logging-Output.html#Logging-Output">Logging Output</a>).
</p>
</dd>
</dl>

<p>Flushing <code>sys.stdout</code> or <code>sys.stderr</code> will automatically
call this function for the relevant stream.
</p></dd></dl>

<a name="index-gdb_002etarget_005fcharset"></a>
<dl>
<dt><a name="index-gdb_002etarget_005fcharset-1"></a>Function: <strong>gdb.target_charset</strong> <em>()</em></dt>
<dd><p>Return the name of the current target character set (see <a href="Character-Sets.html#Character-Sets">Character Sets</a>).  This differs from <code>gdb.parameter('target-charset')</code> in
that &lsquo;<samp>auto</samp>&rsquo; is never returned.
</p></dd></dl>

<a name="index-gdb_002etarget_005fwide_005fcharset"></a>
<dl>
<dt><a name="index-gdb_002etarget_005fwide_005fcharset-1"></a>Function: <strong>gdb.target_wide_charset</strong> <em>()</em></dt>
<dd><p>Return the name of the current target wide character set
(see <a href="Character-Sets.html#Character-Sets">Character Sets</a>).  This differs from
<code>gdb.parameter('target-wide-charset')</code> in that &lsquo;<samp>auto</samp>&rsquo; is
never returned.
</p></dd></dl>

<a name="index-gdb_002esolib_005fname"></a>
<dl>
<dt><a name="index-gdb_002esolib_005fname-1"></a>Function: <strong>gdb.solib_name</strong> <em>(address)</em></dt>
<dd><p>Return the name of the shared library holding the given <var>address</var>
as a string, or <code>None</code>.
</p></dd></dl>

<a name="index-gdb_002edecode_005fline"></a>
<dl>
<dt><a name="index-gdb_002edecode_005fline-1"></a>Function: <strong>gdb.decode_line</strong> <em><span class="roman">[</span>expression<span class="roman">]</span></em></dt>
<dd><p>Return locations of the line specified by <var>expression</var>, or of the
current line if no argument was given.  This function returns a Python
tuple containing two elements.  The first element contains a string
holding any unparsed section of <var>expression</var> (or <code>None</code> if
the expression has been fully parsed).  The second element contains
either <code>None</code> or another tuple that contains all the locations
that match the expression represented as <code>gdb.Symtab_and_line</code>
objects (see <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>).  If <var>expression</var> is
provided, it is decoded the way that <small>GDB</small>&rsquo;s inbuilt
<code>break</code> or <code>edit</code> commands do (see <a href="Specify-Location.html#Specify-Location">Specify Location</a>).
</p></dd></dl>

<dl>
<dt><a name="index-gdb_002eprompt_005fhook"></a>Function: <strong>gdb.prompt_hook</strong> <em>(current_prompt)</em></dt>
<dd><a name="prompt_005fhook"></a>
<p>If <var>prompt_hook</var> is callable, <small>GDB</small> will call the method
assigned to this operation before a prompt is displayed by
<small>GDB</small>.
</p>
<p>The parameter <code>current_prompt</code> contains the current <small>GDB</small> 
prompt.  This method must return a Python string, or <code>None</code>.  If
a string is returned, the <small>GDB</small> prompt will be set to that
string.  If <code>None</code> is returned, <small>GDB</small> will continue to use
the current prompt.
</p>
<p>Some prompts cannot be substituted in <small>GDB</small>.  Secondary prompts
such as those used by readline for command input, and annotation
related prompts are prohibited from being changed.
</p></dd></dl>

<hr>
<div class="header">
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Next: <a href="Exception-Handling.html#Exception-Handling" accesskey="n" rel="next">Exception Handling</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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